Australia’s sheltered coastal waters are increasingly being considered as providing important opportunities for aquaculture. The main species being farmed are Atlantic salmon, southern bluefin tuna, rock oysters, pearl oysters, mussels, prawns and abalone. These species are farmed in land-based and sea-based facilities, both of which have a range of environmental risks. There are four main areas of environmental concern: the potential for spread of diseases and parasites, the impacts of the facilities and supporting infrastructure, the interaction with wildlife, and the source and sustainability of wild stocks (if required) and feed. Key issues of environmental concern are diseases that can be harboured in, and spread from, both types of facilities; treatment and impacts of wastes, particularly feed and faeces; intensification of infrastructure in sensitive habitats; and effects on species that may become dependent on the structures or waste discharges.
In Australian waters, evidence indicates that both land-based and sea-based aquaculture has been the source of a number of major outbreaks of diseases in wild populations. The resulting impacts have been ecologically significant and will leave a lasting imprint on some of the affected ecosystems. In addition to disease outbreaks, there have been issues associated with use of chemicals, and impacts on threatened species such as sharks and seals. This is consistent with overseas experience of aquaculture impacts.60 However, given appropriate levels of management and verification, the impacts of aquaculture facilities can be constrained to a minimal and acceptable level, bringing the aquaculture industry in Australia into line with other modern farming practices to produce wealth from the ocean with minimal environmental degradation.
Australia has a long history of aquaculture in the estuaries of the east coast. The Sydney rock oyster (shell) was harvested for use as lime in cement production in Sydney in the 1800s, but this quickly depleted the local oyster beds. The earliest marine farming operations of oysters were subsequently established by Thomas Holt in Gwawley Bay (Georges River) in 1872, in response to the depletion of wild oysters. The industry was heavily focused on the Hawkesbury River in its early years, but declining water conditions and high levels of diseases have now almost eliminated production from this estuary. Oyster farming in New South Wales has now diversified to include the Pacific and flat oyster, on selected sites held under some 3200 aquaculture leases, with a total current area of approximately 4300 hectares.61 The main oyster-producing areas are located away from urban areas. Commercial production in New South Wales occurs in 41 estuaries between Eden in the south and the Tweed River in the north, although Wallis Lake (on the north coast) is now the main Sydney rock oyster-producing area.
In the first 75 years of the New South Wales oyster industry, production of the endemic Sydney rock oyster grew to about 60 million oysters per year. In the subsequent 25 years, production increased to about 175 million oysters per year, peaking in 1977, and then trended downwards to the current 70 million oysters per year—less than half the production of the industry at its peak. Disease and environmental issues remain significant problems for this industry.
The statewide reduction in production is related to the impact of land-based sources of pollution (from urbanised areas and agriculture), and to an extensive and diverse set of waterborne diseases in farmed oysters, including viral and bacterial infections, protozoa and flatworms.62 These accelerating issues have resulted in a much greater emphasis on the development of land management in river catchments that recognises the need for high water quality in oyster-growing areas.
Disease issues in the oyster industry are also concerns for wild oyster populations. They include the potential transmission of diseases between the estuaries, related to industry practices, and possible maintenance of diseases in the wild population that might otherwise naturally dissipate to background levels. Oysters (wild and farmed) have an important role in estuaries, filtering water and feeding on plankton and other fine debris to clarify the water. Although the role of the intensive aquaculture system in transporting and spreading disease among the wild population or to other molluscs is unclear, these are important ecological impact issues for these estuaries and coastal waters. Also of concern is the spread of the Pacific oyster—this species is endemic to Japan and farmed in several states, and has developed many naturalised populations along the east coast. The ecological impact of this introduced species is uncertain, but is likely to be significant. Where its populations have become established, it is likely to compete with native species (including the Sydney rock oyster) for space and food, and possibly has impacts on a range of other sedentary species that also inhabit the estuaries of New South Wales.
Abalone aquaculture is a recent initiative, mainly undertaken in Tasmania, Victoria and South Australia, where the most substantial natural populations of abalone also occur. In 2008–09, around 640 tonnes of abalone were produced from the aquaculture facilities in these states.
Two species are farmed—greenlip abalone (Haliotis laevigata) and blacklip abalone (H. rubra)—as well as a hybrid of these species, in land-based and sea-based farming systems. The two systems have very different siting and infrastructure requirements, and a different range of associated environmental risks. For example, in land-based tank systems, the growing abalone are fed on an artificial diet, require large volumes of fresh sea water and produce a large volume of wastewater. In sea-based systems, the growing abalone are fed on natural macroalgae (which may be harvested locally by hand), require only modest current flows of high-quality sea water and produce little waste. However, in both cases, high densities of individuals can lead to the risk of outbreaks of diseases that can very quickly (within days) become difficult to treat and control (Box 6.8).